We propose to determine the structure of a complex between the E. coli chaperonin GroEL (Cpn60) and the gp 5 capsid protein of bacteriophage HK97. Suitable crystals have been obtained, they can be flash frozen at -160oC and preliminary data have been collected at CHESS to 3.6 [unreadable] resolution. Chaperonins, including members of the Cpn60 family are a ubiuitous class of proteins that assist in protein folding in vivo by binding to partially folded proteins; this prevents aggregation and may unfold off pathway intermediates and direct them back into the correct folding pathway. The gp 5 protein is a natural substrate of GroEL; it is the major capsid protein in the ocosahedral head of the lambdoid bacteriophage HK97. The unit cell parameters are: a = 138.0,b = 270.9, c = 155.4 [unreadable], b = 101.4o. The Space Group is P2. The asymmetric unit contains one 14mer of GroEL and one subunit of the HK97 gp5 protein, i.e. it is the smallest asymmetric unti that could contain a complete complex of this type.